The present invention relates generally to the screening of products for the presence of latent defects which would be exposed by normal usage and transportation during the life of the products. More particularly, the present invention relates to a new and improved method for environmentally stress screening products, by subjecting the products to thermal and vibrational stimuli, optimally simultaneously. Other stresses such as electric power cycling, voltage changes, and input power frequency variations may be introduced to the products being screened, and the products can be monitored to determine if the products function and continue to function as expected. It has been found that these combined thermal and vibrational stimuli, when combined with simultaneous rigorous functional monitoring, enable root cause corrective action to be implemented to remedy the cause of latent defects uncovered during the screening. Environmental screening in accordance with the present invention can lead to significant product quality improvements, production cost reductions, reduced warranty repair expense, increased customer satisfaction, and increased market share.
Applications of the current state of the art in environmental screening is very energy, time, and effort intensive, thereby rendering such screening very expensive. Existing screening techniques utilize temperature cycling within a chamber which, in more advanced cases, also has a product vibrating device within it. The chamber itself and the vibrating device act as thermal loads on the chamber, thus requiting more energy to heat and cool the electronic product being tested through the desired thermal cycles. Both the vibrating device and the thermal loads react to temperature changes, radiating and/or absorbing thermal energy in response to changes in ambient temperature. Accordingly, changing the temperature of the chamber and the vibrating device naturally requires more energy than if just the product were to be cycled through temperature changes. In some cases, the chamber and vibrating device consume the majority of the energy costs to no useful end. Additionally, cycling the chamber and vibrating device through various temperature changes requires more time than if just the product were to be cycled.
As an alternative to screening products by cycling them through different temperatures in a single, variable-temperature chamber, products may be successively moved into and subjected to multiple temperature-controlled chambers which are maintained at different fixed temperatures. Existing "thermal shock chambers" shuttle products between temperature-controlled chambers to subject the products to differing extremes of temperature. However, when the chambers are opened or closed to insert and remove products being screened, thermal energy can be exchanged with adjacent chambers and/or the ambient environment, and energy will be wastefully consumed in restoring the desired temperature within the chambers. Further, manual intervention is required if the products to be screened must be connected to or disconnected from vibrating devices or other testing devices, and additional thermal energy will be lost to the ambient environment while the chambers are open to allow the connections to be made.
Where screening for defects susceptible to vibration is desirable, energy efficiency concerns are magnified. If a temperature-controlled chamber is equipped with a vibrating device, such as a shaker table, when the chamber is heated and/or cooled, the vibrating device must also be heated or cooled. Accordingly, reaching and/or restoring the desired temperature in the chamber requires additional energy to heat and/or cool the vibrating device, as described above. Further, repeated heating or cooling of the screening apparatus may have detrimental effects on the reliability of the vibrating device due to material fatigue as well as potentially nonuniform thermal expansion and contraction of parts and other effects, which may be detrimental to the operability of moving parts.
In sum, existing environmental screening systems consume energy and time inefficiently. Existing systems also require manual intervention to connect products to testing devices for testing vibrational and other stimuli. It is because of these and other background considerations that the present invention has evolved.